Porpoises are marine mammals closely related to dolphins and whales, belonging to the group of toothed cetaceans. Understanding their diet provides insight into their ecological role within ocean ecosystems, helping scientists assess their health, energy requirements, and how they interact with other marine life. It also reveals how these animals adapt to their environment and food availability.
Primary Food Sources
Porpoises primarily feed on a variety of fish, along with cephalopods such as squid and octopus, and occasionally crustaceans. Their diet commonly includes non-spiny schooling fish like herring, cod, whiting, mackerel, and sardines. They also consume sandeels, gobies, and sprat, often targeting small, shoaling species found in both open waters and near the seabed. Harbor porpoises, for instance, are known to eat about 10 percent of their body weight each day, which can amount to around five kilograms of fish.
These marine predators employ echolocation, a sophisticated biosonar system, to locate and capture prey in their underwater environment. They emit high-frequency clicks, typically within the 110-150 kHz range, and interpret the echoes that return from objects. As a porpoise closes in on its target, the interval between these clicks shortens, escalating into a rapid series of clicks known as a “feeding buzz” just before capture. This allows them to precisely track and seize their agile prey.
Many porpoise species are benthic foragers, meaning they search for food on or very close to the seafloor, but they also hunt in the pelagic (open water) zone. Their hunting requires significant energy, with a harbor porpoise needing to catch approximately 2,000 small fish daily to meet its energetic demands.
Dietary Variations Across Species
While porpoises share general dietary preferences, their specific food choices exhibit considerable variation across different species, geographic locations, and with changing seasons. The availability of prey in local environments dictates much of this diversity.
For instance, the diet of harbor porpoises can differ significantly depending on whether they inhabit coastal regions or the open sea. In the coastal areas of the Netherlands and the Baltic Sea, they frequently consume small benthic fish like gobies and occasionally small flatfish. Conversely, in the open North Sea, their diet shifts to include more herring, sprat, and mackerel. Studies in Scottish waters show a preference for whiting and sandeels. The diet also changes with age; juvenile porpoises often consume smaller, leaner fish such as gobies, while adults typically target larger fish and a broader variety of species.
Dall’s porpoises, found in the North Pacific Ocean, exhibit a different dietary pattern. They dive to depths of up to 1,640 feet to feed on small schooling fish like anchovies, herring, hake, capelin, and lanternfish, as well as mid- and deep-water smelts. Cephalopods, including squid and octopus, are also regular prey for Dall’s porpoises, with crustaceans occasionally supplementing their diet. They often forage at night, capitalizing on prey that migrates closer to the surface.
Ecological Significance and Conservation
Porpoises hold a role in marine food webs as predators, influencing the populations of the fish and invertebrates they consume and contributing to the balance of marine ecosystems. Their feeding activities can also play a part in nutrient cycling within the ocean, as they dive to feed and release nutrients closer to the surface.
Despite their ecological contributions, porpoises face challenges that impact their food sources and, consequently, their survival. Overfishing poses a significant threat, as it depletes the fish populations that porpoises rely on for sustenance. Many of the fish species that constitute a porpoise’s primary diet are also commercially targeted, leading to competition for resources. This reduction in prey availability can lead to emaciation and increased mortality among porpoise populations.
Accidental entanglement in fishing gear, known as bycatch, is another threat directly linked to their feeding habits. Porpoises can become trapped in nets while foraging, leading to suffocation. Additionally, habitat degradation from pollution, including chemical and noise pollution, impacts their ability to find food. Noise from human activities, such as boat traffic, can disrupt their echolocation and hunting success, forcing them to spend more time searching for food. Given their relatively slow reproductive rate, typically one calf every two years, these threats make it difficult for porpoise populations to recover from declines.